OF INEUMATICS. 195 



LECT. 

 VI. 



LECTURE VI. 



OF PNEUMATICS. 



THIS science treats of the nature, weight, pressure, 

 and spring of the air, and the effects arising therefrom. 



The air is that thin, transparent, fluid body in which The pro- 

 we live and breathe. It encompasses the whole earth ^ e r rtl< 

 to a considerable height ; and, together with the clouds 

 and vapours that float in it, is called the atmosphere. 

 The air is justly reckoned among the number of fluids, 

 because it has all the properties by which a fluid is dis- 

 tinguished. For, it yields to the least force impressed, 

 its parts are easily moved among one another, it presses 

 according to its perpendicular height, and its pressure 

 is every way equal. 



That the air is a fluid, consisting of such particles as 

 have no cohesion betwixt them, but easily glide over one 

 another, and yield to the slightest impression, appears 



from that ease and freedom with which animals breathe 







in it, and move through it without any difficulty or sen- 

 sible resistance. 



But it differs from all other fluids in the four follow- 

 ing particulars. 1. It can be compressed into a much 

 less space than what it naturally possesses, which no 

 other fluid can. 2. It cannot be congealed or fixed, as 

 other fluids may. 3. It is of a different density in every 

 part, upward from the earth's surface, decreasing in its 

 weight, bulk for bulk, the higher it rises ; and therefore 

 must also decrease in density. 4. It is of an elastic or 

 springy nature, and the force of its spring is equal to 

 its weight. 



o 2 



